The main objective of this project is to determine the mode of action of thyroid hormones (T4 and T3). Studies are designed to investigate factors controlling transfer of thyroid hormone from blood to cells, their intracellular metabolism, interactions with nuclear receptors, and finally, their effect on the induction of specific messenger RNAs. The biologic materials used in these studies are: (1) rat pituitary tumors producing growth hormone (GH) and prolactin (PRL) the synthesis of which is known to be modulated by thyroid hormone; (2) rat pituitaries in vivo and in vitro; (3) rat liver; and (4) cells (leukocytes and fibroblasts) from patients with inherited resistance to the action of thyroid hormone. Radioiodide labeled hormones are used for the study of the kinetics and metabolism of T4 and T3. The effect of thyroid hormones and their analogues on GH and PRL synthesis are measured by 3H-leucine pulse labeling technique and immunoprecipitation using specific anti-rat GH and PRL antiserum. Accumulations of GH and PRL mRNAs are quanitated by translation in cell free system using a nuclease treated rabbit reticulocyte lysate and hybridization technique to 3H-labeled complementary DNA. The effect of thyroid hormones and their analogues on protein synthesis and the induction of specific mRNAs are correlated. Important steps in the mediation of thyroid hormone action are verified by their absence in patients with inherited resistance to thyroid hormone. It is also proposed to study (1) the anamnestic response of PRL to estrogens; (2) changes in T3 receptor capacity in liver and pituitary and their correlation to accumulation of specific mRNAs; and (3) the effects of thyroid hormone on nuclear RNA; and to characterize (4) the immunoreactive but biologically inactive TSH induced during treatment with human GH, and (5) PRL mRNA in human prolactin producing adenomas.